Categorization underlies many of our abilities to integrate, understand, and predict our experiences. Central to many categories are symbols that represent objects, events, or even other concepts. Yet individuals with severe disabilities often demonstrate pronounced difficulties with symbol acquisition or manipulation. These deficits raise fundamental questions: What is the nature of symbolic behavior? What does symbolic behavior allow humans to accomplish that could not be accomplished otherwise? Also provoked are questions about the nature of severe disabilities: Are individuals with very limited language skills entirely "non-symbolic?" What is the relation between symbolic functioning, concept learning, and other aspects of cognition in this population? Answers to these questions have remained elusive, due in part to lack of (1) general agreement as to what constitutes symbolic functioning and (2) appropriate, conventionalized methodology for addressing these questions, particularly in individuals with severe disabilities. Work accomplished in our past research has set the stage for a cross- disciplinary effort to make progress on these important, but difficult issues. We propose to conduct the first large-scale study of stimulus equivalence development, a fundamental prerequisite for symbolic functioning and category learning, in participants who are non-verbal or have very limited language. Is stimulus equivalence routinely demonstrable in these individuals? Can positive equivalence outcomes be related to language status or other participant characteristic? Can higher-order equivalence be demonstrated in these individuals, if the appropriate procedure are used? Complementary research will use evoked potential methods to assess the adequacy of laboratory-defined equivalence relations as a model for extra-experimentally-defined category learning. In addition, studies are planned to show how the methods used in this study might be applied in educational settings.